The science of nanotechnology is advancing rapidly, largely as a result of the enormous number of investigators and generous funding, however, examples of practical and affordable implementations of nanotechnology are not as prevalent. There are several challenges uniquely linked to nanomaterials and two critical issues stand out. First, manufacturing methods are required to produce nanomaterials reproducibly, with narrow particle size distributions, at volume, and at appropriate cost points. Second, manufacturing methods are required to produce uniformly surface functionalized nanomaterials that allows them to be integrated into the desired matrix with the retention of performance. The dimensional attributes of nanoparticles that offer such remarkable performance also bear, at times, overwhelming handling difficulties. The inability to successfully meet these challenges has crippled many efforts to bring nanomaterial opportunities to fruition.
In the biomedical field, nanotechnology has begun to deliver on its extraordinary potential; for example, nanomaterials have been shown to improve bone integration and increase the healing rate after surgery. In the context of dental applications, polymer nanocomposites dramatically enhance the strength, hardness, and durability of dental restoratives, and facilitate a wide array of commercial applications that emulate the remarkable properties of dental hard tissues (enamel and dentin). There are appreciable challenges in the manufacture and integration of such materials. Under the best circumstances, traditional solvent-based or sol-gel paths realize modest yields, with the environmental liability of waste hydrocarbons. Not surprisingly, the incorporation of these high surface area particles into the preferred monomers and polymers is not trivial; the mismatch in interfacial energy between nanomaterials and biomedical polymers often preclude their integration into these materials.
In applications where nanoparticles and/or micron sized particles are used together with polymers, prepolymers, oligomers and monomers and other hardenable or nonhardenable resins (hereafter referred to collectively as resins), the degree of dispersion of the particles is particularly important with respect to the performance attributes of the final article being produced from the dispersion. Poorly dispersed particles will result in inhomogeneities within the resin and a concomitant degradation in performance attributes. In resins that are polymerized, hardened or cured via electromagnetic radiation, for example, ultraviolet, visible, electron beam or x-radiation, the homogeneity is of particular importance since scattering of the incident radiation by the particles may limit absorption and reduce the cure rate and/or the cure depth of the article. In this same context, the presence of nano sized and/or micron sized particles may opacify the article and prevent penetration of the curing radiation. It is therefore of interest to prepare particle compositions that have the same refractive index as the resin in which they are dispersed. Because the particles and resin have the same refractive index, the incident radiation is not highly scattered and such dispersions appear transparent or translucent. This increases the penetration depth of the curing radiation and increases the cure rate and cure depth. Still further, if the refractive index of the particles is designed to have the same refractive index of the resin after curing (since the refractive index of most resins changes slightly upon curing) the final article may be transparent or translucent and have desirable aesthetic qualities. Still further, transparent or translucent resins that have a high loading of hard and strong inorganic particulates have a variety of uses in optical coatings and composites, often having superior mechanical strength and scratch resistance compared to the unloaded resin.
There is a need therefore to produce particulates or fillers in which the refractive index can be precisely controlled so that they may be index matched to a variety of resins and so that transparent or translucent articles may be produced with high performance attributes.
U.S. Pat. No. 4,217,264 to Mabie et al., discloses a microporous filler for dental composite resin restorations which gives greatly improved finishability, system nontoxicity and x-ray opacification. These fillers are prepared from frits obtained by the low temperature calcination of gelled inorganic polymers followed by a pulsed high heat treatment. Mabie describes microporous glassy fillers that have a median refractive index between about 1.51 and about 1.64, the microporous glassy filler comprising a gelled, calcined sol comprised of refractory inorganic oxides including silica and alumina, and at least one nontoxic x-ray opacifying agent selected from the group consisting of zirconium, hafnium, tantalum and tin oxides. Mabie describes metal oxide mixtures that share the same refractive index as the polymers and/or monomers used in dental restoratives. There is a problem, however, in that such mixed oxides have a high surface area and are brittle, and further the mixed oxides are nonhomogeneous and have a broad refractive index dispersion and do not provide adequate transparency.
U.S. Pat. No. 4,503,169 to Randklev discloses radiopaque, low visual opacity, non-vitreous microparticles that consist primarily of silica and zirconia. Randklev discloses (column 5, line 52) that “other metal oxides which may not themselves provide sufficient radiopacity can, if desired, be included in the microparticles of the present invention”. However, the materials of Randklev have a low visual transparency and further Randklev does not describe methods for achieving high visual transparency. Still further, the compositions described by Randklev require one or more pulsed heat treatments (column 6, lines 33-65) in order to remove organic impurities and to convert the precursor materials to a mixed oxide composition. It would be advantageous to produce mixed oxides that are free of organic impurities and do not require expensive heat treatments to form the mixed oxides.
U.S. Pat. No. 4,764,497 to Yuassa et al. discloses an amorphous, spherical inorganic compound with a particle size of 0.1 to 1.0 um that consist of a mixture of silica-zirconia, silica-titania and silica-alumina mixed oxides, among others. The silica-alumina mixed oxides are prepared by reacting an alkoxysilane with an organo-alumina complex in organic solvent in the presence of a small amount of water to catalyse the reaction. There is a problem, however, in that the reactivity of the two reagents is vastly different and homogeneous mixed oxides are difficult to prepare. The highest alumina loading achieved is just 6.3 mole % alumina and the corresponding refractive index is only 1.46-1.47, see examples 3, 53-55 and 58-60. Further, formation of the mixed oxide by this method requires processing temperatures of about 1000° C.
U.S. Pat. No. 7,030,049 to Rusin et al. discloses non vitreous microparticles prepared by a sol-gel method in which an aqueous or organic dispersion or sol of amorphous silicon oxide is mixed with an aqueous or organic dispersion, sol, or solution of a radiopacifying metal oxide, or precursor organic or inorganic compound, the microparticles being substantially free of crystalline microregions or inhomogeneities detectable via powder x-ray diffraction. In column 28, Rusin discloses mixed oxides comprising SiO2—B2O3—La2O3—Al2O3 (fillers c-e) and SiO2—La2O3—Al2O3 (fillers l-m). There is a problem, however, in that the fillers of Rusin et al. inevitably contain high amounts of heavy metal radiopacifying agents and/or melt flux reagents such as boron oxide that decrease the hardness and strength of the mixed oxide and further add to its cost. The melt flux reagents lead to a sintering of the mixed oxide and therefore a high amount of energy is required to reduce the particle size of the mixed oxide.
U.S. Pat. No. 7,160,528 to Rusin et al. discloses a melt derived filler comprising 5-25% by weight aluminum oxide, 10-3 5% by weight boron oxide, 15-50% by weight lanthanum oxide, and 20-50% by weight silicon oxide. There is a problem, however, in that melt derived fillers require high temperature processing and energy intensive diminution of the glass formed.
U.S. Pat. No. 7,335,250 to Burtscher et al. discloses a dental composite comprising at least one nanoparticulate mixed oxide (a) of SiO2 with x-ray opaque metal oxides of one or more elements selected from the group consisting of Y, La, Ta, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu which has been prepared by flame spraying and wherein the mixed oxides have at least one of the following features: a) an amorphous structure, b) a homogeneous element distribution, c) a very low organic content, d) an x-ray opacity which can be varied, e) an index of refraction which can be varied, and f) a spherical particle shape. The patent does not mention silica-alumina mixed oxides.